In low power (e.g. wireless) communication systems using TDMA, timing is a critical parameter which significantly affects how timeslots can be effectively allocated and utilized.
Usually a timeslot is assigned by a central node or base station, and a client (e.g. a sensor node) is required to ensure its communication with the base station is within its allocated timeslot. This requirement therefore imparts constraints on the precision of clocks that can be used by the client. However, in applications where it is desirable to employ low power and/or low cost clients (e.g. as disposable sensor nodes), it is also desirable to employ simpler and cheaper clocks, which are generally less precise.
To date this problem has been overcome by ensuring that the client wakes up in advance of its allocated slot (as determined by the clock on the client) while the base station is configured to begin transmitting at the allocated start time (according to its own, usually more precise clock) so that the client will always be awake when the base station transmission begins. The disadvantage with this approach is that, typically, the client will wake up early and may have to wait some time before the base station transmission begins. This wasted time consumes power and is therefore highly undesirable in such low cost devices.
Even when a client node has a clock of the same or similar precision as the base station, it may have to wake up occasionally to synchronise its clock with the base station if it has a very long sleep period. This is particularly important when there are other clients on the same network using other timeslots. This system therefore places the onus on the client to wake up at known time intervals in order to keep in sync with the base station and this constitutes an additional power expenditure.
It is therefore an aim of the present invention to provide a TDMA-based communication method and system that addresses the aforementioned problems.